The present invention relates to copying machines in general, and more particularly to improvements in apparatus for maintaining sheets or webs of photosensitive material in a predetermined plane during exposure to light at the copying station of a copying machine. Still more particularly, the invention relates to improvements in apparatus for transferring and maintaining sheets or webs of photosensitive material, especially relatively large sheets or webs, in a predetermined (copying) plane by resorting to suction generating means.
Photosensitive material which is exposed to light in a photographic copying machine is normally transported in the form of elongated webs. Such webs must be supported and guided to insure convenient stepwise transport for placing of successive coherent portions or sheets of the web into register with various instrumentalities at the copying station. As a rule, the areas to be exposed to light must be moved into a predetermined plane in order to insure the making of sharp exposures. The transport and proper positioning of portions of relatively small (narrow) webs present no problems; such webs can be readily held in requisite position by mechanical clamping, holding and/or pressing devices. However, mechanical devices are unsatisfactory for reliable positioning of relatively large webs (e.g., webs consisting of coherent sheets having a width of up to 30 centimeters) in a predetermined plane during exposure to copying light.
German patents Nos. 1,146,741, 1,065,271 and 1,187,122 disclose apparatus with means for attracting sheets to a supporting surface which is located in a predetermined plane by resorting to suction generating devices. A drawback of the patented apparatus is that proper retention of sheets in preselected planes can be insured only by resorting to large-capacity pumps in order to guarantee predictable transfer of sheets from the transporting plane (i.e., from the path along which the sheets are transported toward and beyond the copying station) to the copying plane. A high-capacity pump (and/or its motor) invariably produces considerable noise which is bothersome to the attendants. Moreover, the energy requirements of suction generating means are extremely high and the sheet (e.g., a portion of a continuous web) which is transferred from the transporting plane into the copying plane is likely to develop folds, creases, pleats or other surface irregularities which affect the quality of reproductions. The likelihood of development of folds or the like is especially pronounced immediately after the copying machine is started and also when the machine is designed in such a way that the sheet is illuminated from below. In such machines, the sheet is likely to form a relatively small loop which hangs downwardly beyond the positive plane. As a rule, the marginal portions of the sheet are attracted first whereby the median portion of the sheet exhibits a bulge or fold, i.e., the median portion does not lie flat against the platform which serves to maintain the sheet in an optimum position for exposure to light.